Fluorescent screen for electron-ray tubes operating at low voltage



Jan. 12, 1960 A. LIEB 2,921,201 FLUORESCENT SCREEN FOR ELECTRON-RAY TUBES OPERATING AT LOW VOLTAGE Filed Dec. 15, 1955 GLASS MIXTURE 0F FLUORESCENT SUBSTANCE AND NON-FLUORESCENT METAL OXIDES INVENTOR ALBERT LIEB ATTORNEY "FLUORESCENT SCREEN FOR ELECTRON-RAY TUBES OPERATING AT LOW VOLTAGE ---.Application December 15, 1955,'Serial No. 553,403

: 2 Claims. (Cl. 250-80) 1 This invention relates to fluorescent screens for electron-ray 'tubes operating at low voltages, in particular for indicator tubes. On the fluorescent screen of such electron-ray tubes there result, in the case of a small secondary emission of'the fluorescent substance, charging phenomena which disturb the desired discharge process. Thus, for example, with indicator tubes in which the fluorescent substance is arranged, as a rule, on a conductive base, in particular at operating voltages below 200 volts, there can easily appear disturbing fluorescent pattern phenomena on the fluorescent screen, e.g. it sponsors the formation of dark portions or there can appear a complete extinguishing of the fluorescent pattern.

The fluorescent substances for indicator tubes should have a suitable emission color, an extensive insensitivity of the fluorescent substance properties with respect to the manufacturing conditions of'the tube, in particular with respect'to the necessary annealing processes to which the fluorescent screen is subjected, and a high velocity of light emission at low electron velocities. With indicator tubes the emission of a tightly limited spectral range, approximately near the maximum of the human sensitivity of the eyes, is most suitable. In the case of the conventional manganese-activated zinc-silicate fluorescent substance, which best complies with these requirements, there result disturbing charging phenomena when excited by electrons of low velocity, because of an insuflicient secondary emission. Also other fluorescent substances, such as aluminum-oxide which produces a green light, and which sufliciently meets the above-requirements, produce an insuflicient secondary emission under these operating conditions.

The disturbing charging phenomena are also the result of the special arrangement of the fluorescent substance on the base. When employing an electrically conductive metal base with a smooth surface then this base will sponsor the formation of stronger disturbing charges than when employing a base with a surface roughened e.g. by sand blast. Furthermore, a fluorescent substance deposited loosely upon the base will be charged to a stronger extent than a fluorescent substance which is firmly deposited upon the base. A loosely applied coating of the fluorescent substance will result especially when the fluorescent substance is applied by means of a spraying method. The spraying upon a smooth base represents a particularly economical way of manufacturing the fluorescent screen, but when employing this economical method the charging eifects will be noticed most strongly.

A well known aid for increasing the secondary emission is adding oxides of alkali metals and alkaline earth metals. The addition of the alkali oxides or alkaline earth oxides, however, brings about either a still insuflicient increase of the secondary emission, or a too strong diminution of the luminous density.

Further, there are known types of fluorescent screens in which to the fluorescent substance there are added metal oxides with a dark body color, such as nickel oxide,

Patented Jan. 12, 1960 ice , for'the purpose of increasing the optical contrast or for absorbing the secondary electrons disturbing the optical contrast. In this particular case, and since there is achieved the opposite elfect, a higher luminous density and the freedom from disturbances of the fluorescent pat-* tern at low fluorescent screen voltages issacrificed, in favor of the contrast.

In addition, fluorescent screens are known which consist of mixtures of various fluorescent substances. Thus, for example, for reducing the decrease in brightness'the zinc silicate, 'which is mostly employed with indicator tubes, is provided with additives of fluorescent zinc'oxide, the brightness of which does not decrease in proportion with'the service life. By employing the zinc-oxide fluorescent-substance additive the brightness of the fluorescent screen is prevented from dropping below a certain lowervalue during a predetermined period of service;

The zinc-oxide fluorescent-substance, when being excited by electrons of a low velocity, shows a bluish emission color, and a strong white level.

By adding the zinc oxide to the zinc silicate there results the disadvantage that the emission of the green fluorescent zinc silicate is changed in an unfavourable manner into a pale fluorescent green-bluish emission with a strong white level. Thereby the reading is rendered more diflicult, especially in the presence of room light. Another difficulty appears, especially after'a longer period of service, because then the zinc oxide, because of its better life properties, more strongly affects the light emission of the fluorescent substance mixture. The emission of the zinc oxide v.in the temperature range from 20-200 C., unlike the manganese-activated zinc silicate, still varies with the temperature of the fluorescent substance. Due to the different temperature dependency of the emission of these fluorescent substances there result disturbing displacements of the emission color also during the switching-on process.

For avoiding the disturbing phenomena with fluorescent substances, it is suggested by the invention to add to the fluorescent substance a non-fluorescent zinc oxide, viz. zinc oxide without any specially prepared luminous centers, or otherwise titanium dioxide. These two oxides, especially the titanium dioxide, result in a considerable increase of the secondary emission at low fluorescent screen voltages, and their body color is such that the luminous efliciency of the fluorescent substance will only be slightly reduced, and they are sufficiently stable in chemical as well as in thermal respects.

The addition of the non-fluorescent zinc oxide, when compared with the already known employment of fluorescent zinc oxide as an additive, bears the advantage that the emission color of the chosen fluorescent substance will be maintained. For the purpose of confirming the selection of these substances, a number of oxides, such as MgO, BeO, CaO, SrO, CdO, BaO, Ta O Nb O SiO A1 0 ZrO ZnO, were added to the manganese-activated zinc-silicate fluorescent substance, or to the green fluorescent aluminum oxide in a quantitative proportion. Thereby it was proved that the oxides of the zinc, and in particular those of the titanium, which were added in accordance with the invention, caused the substantially strongest reduction of the disturbances of the fluorescent pattern, and provided the highest secondary electron efliciency at a low fluorescent screen voltage. The desired proportions of additive zinc oxides is from 25 to 50% with respect to the fluorescent coating and of titanium oxide between 25% and 50%.

In the single figure of the drawing is illustrated an example of a portion of a cathode ray tube screen showing the invention. On the screen face 1 is provided a the tube, such as annealing treatment of the fluorescent screen, and the like, whereas the previously insuflicient secondary emission will be substantially increased. Av

fluorescent screen manufactured in this way will also provide an interference-free fluorescent pattern when operat-- ing at very. low fluorescent screen voltages. Further more, in this case also simple and, consequently, econom ically advantageous manufacturing methods, such as the spraying upon a smooth surface acting as the base of the fluorescent screen, can be accomplished without any difficulties. p

From the foregoing description it will be seen that fluorescent screen compositions may be made in accordance, with the following examples.

Example 1:

Fluorescent aluminum oxide-75 to 50% Non-fluorescent zinc oxide25 to 50% Example 2:

j Fluorescent aluminum oxide-75 to 50% Titanium oxide-25 to 50% Example 3:

Manganese activated zinc silicate-75 to 50% Non-fluorescent zinc oxide25 to 50% 4 Example 4:

Manganese activated zinc silicate-75 to 50% Titanium oxideto 50% For obtaining an intimate mixture of the fluorescent substance with the oxides, the invention provides to insert both of them into a suspension agent, where they are either shaken in common or, still better, are ground together. The last-mentioned method results in the added advantage that the grain size will be still smaller, and the degree of mixture will still be improved.

What is claimed is:

1. A fluorescent screen for electron-ray tubes operated by alow voltage, in particular for indicator tubes, comprising a fluorescent substance chosen from the group consisting of fluorescent zinc silicate and green-fluorescent aluminum oxide containing an addition of metal oxides for improving the secondary emission, comprising between 25 and 50% by weight of non-fluorescent.

metal oxides from the group consisting of zinc oxide and titanium dioxide.

2. Afluorescent screen as claimed in claim 1, characterized in that its mixture consists of one part by weight of metal oxide, and two to three parts by weight of fluorescent substance.

References Cited in the file of this patent UNITED STATES PATENTS 

